1. Field of the Invention
The present invention relates to a nonreciprocal circuit device for use in a microwave band such as, for instance, an isolator or a circulator.
2. Description of the Related Art
Generally, a nonreciprocal circuit device, such as a lumped constant isolator or a circulator, has low attenuation of signals in the forward direction and high attenuation of signals in the reverse direction, and is used in a transmission circuit of a communications unit such as, for instance, a mobile telephone.
However, linear distortion in an amplifier integrated into a communications unit causes radiation (spurious emissions, especially at two and three times the fundamental frequency). Since this radiation can cause interference and irregular operation of a power amplifier, it must be kept below a fixed level. Radiation is sometimes prevented by using an amplifier with excellent linearity, or by using an extra filter to attenuate radiated waves.
However, an amplifier with excellent linearity is expensive, and using an extra filter increases the number and cost of components, and in addition, increases the overall size of the communications equipment. For these reasons, these measures cannot easily be used in mobile telephones and the like, where there is a strong demand for smaller and less expensive devices.
On the other hand, a lumped constant isolator functions as a bandpass filter in the forward direction, and consequently it has large attenuation in the forward direction in frequency bands distant from the pass band. It may be envisaged that radiation can be attenuated by utilizing these characteristics to block spurious emissions outside the pass band. However, since conventional isolators were not originally designed to obtain attenuation outside the pass band, their capability for this purpose is limited.
Accordingly, the present applicants devised an experimental isolator (not yet publicly known) which contains a circuit element comprising a low-pass filter. As shown in FIG. 12, this isolator includes an inductor L1 which is a constituent element of a low-pass filter. This inductor L1 is patterned on a dielectric substrate 18 which is provided between a magnetic assembly 4 and a magnet 6, and connected between an input port and a matching capacitor Co'.
Consequently, as shown in the equivalent circuit diagrams of FIG. 13 and FIG. 14, a .pi.-type low-pass filter, comprising the connection of C1-L1-C2, is connected to the input port. Here, since C1 is provided by a part of the capacitance of the matching capacitor Co' of the isolator, it does not need to be provided separately. C2 is formed by externally appending a capacitance to the isolator.
According to the above mentioned isolator containing a low-pass filter, attenuation outside the pass band can be increased, and interference and irregular operation caused by radiation can be prevented. The low-pass filter has a simple constitution and is inexpensive, making an expensive amplifier and an extra filter unnecessary, and enables the device to be made small-scale at low cost.
However, when the above low-pass filter is provided on a dielectric substrate, the magnet is in contact with the dielectric substrate, and consequently there is a concern that the high-frequency material characteristics of the magnet, particularly the tangent .delta. or Dissipation Factor (Dissipation Factor=tangent .delta..times.100[%]), will have an adverse effect on the insertion loss of the isolator.
In general, commercially available mass-produced magnets were not developed for high-frequency components, and they are consequently liable to have a considerable dissipation factor (loss tangent). Therefore, it can be expected that the insertion loss of the isolator will increase when a circuit element on the dielectric substrate is in contact with the magnet. A further problem is that the magnet has a high dielectric constant, making it difficult to form inductance.